1. Field of the Invention
The field of the present invention is systems and methods for tracking eye movement.
2. Background
Four different types of non-contact eye trackers are generally known in the prior art. Of the four, the limbus eye tracker is the simplest and least expensive. The limbus eye tracker aims two pulsed infrared photo-emitters at the iris-sclera boundary on either side of the eye. Because of the difference in color between the iris and sclera, differing amounts of light are reflected depending on the position of the eye relative to the emitter. A pair of infrared detectors picks up the reflected light from the emitters, and the differential signal from the emitters is demodulated and filtered to provide a signal proportional to horizontal eye position. This technique provides a relatively easy-to-use recording method, but it does have substantial drawbacks. First, it is limited to tracking horizontal eye movement, and cannot track vertical eye movement. In addition, the two photo emitters and the pair of infrared detectors must be held immobile with respect to the user's head. Any relative movement between the head and the photo emitters or the infrared detectors can create substantial errors in the tracking data.
Another technique, which is very similar to limbus tracking, tracks the boundary between the pupil and the iris. As with limbus tracking, the emitters and detectors of the apparatus must be immobilized in relation to the head. However, this technique does offer the advantage of being able to track vertical movement of the eye because the pupil is far less covered by the eyelids than the limbus. Another advantage is that the border of the pupil is often sharper than that of the limbus, thus yielding a higher resolution. The advantage of using the pupil boundary for tracking purposes, however, also carries an inherent disadvantage in that the difference in contrast is lower between the pupil and iris than between the iris and sclera, thus making detection of the boundary more difficult.
In another technique, the relative positions between reflections from the cornea and lens are used to track eye movement. When light is shone into an eye, several reflections occur at the boundaries of the lens and cornea. These are the so-called Purkinje images, the first of which is also called the “glint”. An image of light, typically referred to as the “bright-eye”, is simultaneously reflected off the retina. Images of the bright-eye and glint can be recorded, using an infrared sensitive camera, as a very bright spot and a less bright disc, respectively. When the eye is panned horizontally or vertically, the relative positioning of the glint and the center of the bright-eye change with the movement. From the relative positions of the glint and the bright-eye, the direction of gaze can be determined.
A fourth eye tracking technique also uses Purkinje images. Such eye trackers measure the relative displacement of the images formed by the reflection of a light source at the anterior corneal surface and the posterior lens surface, which are known as the 1st and 4th Purkinje images, respectively. Rotation of the eye results in a greater displacement of the 1st Purkinje image relative to the 4th Purkinje image, thereby providing a signal proportional to eye position. The primary drawback of such eye trackers is that they require precise alignment and are not suitable under circumstances where the subject is permitted to have relatively free head movement.